Supermassive black holes

The size of a black hole is defined by the Schwarzschild.

It is related to its mass.

A supermassive black hole can grow larger, only by acquiring more mass.

Greater mass means spacetime is curved over a larger area.

Schwarzschild radius does not denote an actual surface.

It denotes the area beyond which spacetime is curved so much,

the escape velocity exceeds the speed of light.

In other words there is so much mass in such a small volume,

that the gravitational force curves spacetime to such an extent,

that not even light can escape.

A black hole is black, because no light reflects.

Although it is thought that supermassive black holes can grow indefinitely,

there is a theoretical maximum that it will be able to grow,

by the conventional accretion disc method.

When gas is in close proximity to the supermassive black hole,

it becomes gravitationally attracted.

It passes the photon sphere and accretes into a disc.

The material spirals into a black hole, beyond its event horizon.

This is the boundary beyond which light cannot escape.

This upper bound is 15 billion solar mass.

At that point, or usually way before that point has been reached.

The gas in close proximity is exhausted.

The supermassive black hole stops acquiring more mass.

The exception is when occasional star comes too close and is tidally disrupted.

Tidal disruption happen when a star comes too close, and the black hole is pulled apart,

by the black holes tidal forces.

Supermassive black holes can go beyond the upper bound by means of black hole merges.

This follows from galactic mergers, when galaxies merge.

At one point this supermassive black holes, at the centre of both galaxies,

will enter into a gravitational dance.

They will spiral into each other and eventually merge to become one bigger supermassive black hole.

When galaxies merge some stars will get close to supermassive black hole again.

This means that once again material is in close proximity to the black hole.

Some material will spiral into the black hole and once again it can acquire mass.

The current upper bound for the mass of a supermassive black hole,

is merely a reflection of the current universe.

After more time has passed, and more galaxies have merged, the upper bound will increase.

Supermassive black holes will never grow indefinitely, as galactic mergers won't occur indefinitely.

Dark energy, is the force which accelerates the expansion of space.

This force will drive most galaxies apart.

This is why black holes cannot grow indefinitely.